The present invention relates to an electric double layer capacitor and a method for manufacturing the same.
The electric double layer capacitor is a capacitor of large capacity which utilizes the electric energy collected in an electric double layer at the phase boundary between activated charcoal and a liquid electrolyte. Furthermore, this capacitor is capable of instantaneous charge and discharge of electrostatic capacity of the farad order. The activated charcoal heretofore used in the polarizable electrode of a capacitor of this sort is either powdery or fibrous.
The electric double layer capacitor is available in a variety of forms such as plate type, coin type, stack-up type and so on. FIG. 1 is a schematic cross-section view showing an exemplary plate type capacitor. This capacitor comprises rectangular or disk-shaped pellets (polarizable electrodes) 1a,1b each obtainable by compression-molding an activated charcoal powder in the presence of aqueous sulfuric acid, a separator 2 interposed between pellets 1a,1b, collector layers 3a,3b each made of an electrically conductive resin and disposed in contiguity with the outer surfaces of pellets 1a,1b, and an insulating rubber gasket 4 which seals off the interior of the assembly in a gas-tight manner.
FIG. 2 is a schematic cross-section view showing an exemplary coin-type capacitor which comprises disk-shaped polarizable electrodes 5a,5b each made of an electrolyte-saturated web of fibrous activated charcoal, a separator 6, metal-spray layers 7a,7b and metal cases 8a,8b which are sealed together with an insulating rubber composition 9.
FIG. 3 is a schematic cross-section view showing an exemplary stack-up type capacitor comprising a plurality of unit cells stacked up in series. The stack-up capacitor is fabricated by stacking up a number of unit cells 10, each similar to the plate type capacitor, which is necessary to obtain the required voltage rating. The assembly of unit cells is covered with a metal housing (not shown).
All of these conventional electric double layer capacitors are low in the density of electrostatic capacity per unit volume (capacitance/volume). Thus, since the capacitor utilizing an activated charcoal powder molded in the presence of aqueous sulfuric acid is fabricated by mere physical compression of the powder, the packing density of charcoal is 0.5 g/cc at most. The capacitor utilizing fibrous activated carbon is based on a woven fabric and as such has a high dimensional stability which facilitates handling during the manufacturing process and ensures high reliability. However, the voids in the woven texture are a direct cause for a decreased capacity/volume ratio. In the case of the stack-up capacitor, the parts which do not contribute to capacitance, such as the metal housing, account for a large proportion of the total volume so that the capacity/volume ratio of this type of capacitor is low.
Furthermore, the conventional electric double layer capacitor has the disadvantage of large contact resistance, between the polarizable electrode layer and the collector layer and between the collector layer and the metal casing. Thus, since the contact is established by the bonding force of a conductive adhesive or by a compressive pressure from the housing, it is difficult to implement an electric double layer capacitor of low internal resistance. Thus, the conventional capacitor could not be used in large current charge and discharge applications.